Tehran, Iran (CNN) -- Tens of thousands of demonstrators who marched in Tehran in defiance of the Iranian government were largely cleared from the city's streets by nightfall.

Patrolling security forces had battled protesters with batons and tear gas for much of the day.

The wave of people who marched along Revolution Avenue on Monday had remained largely silent as they walked toward the capital's Azadi Square, though at times they clashed with Iranian security forces who tried to disperse the marchers and divert them from the square.

By day's end, dozens of demonstrators had been detained while internet videos showed others had been chased and beaten.

One person was shot and killed during the protests, according to Iran's semiofficial Fars news agency. Several others were injured and listed in serious condition as a result of the shooting, which the Iranian government blamed on "agitators and seditionists."

Uniformed security forces and pro-government Basij militiamen had earlier advanced on crowds who chanted "Death to the dictator!" during demonstrations in the city's Imam Hossein Square -- the planned starting point of a scheduled rally, a witness said.

"We definitely see them as enemies of the revolution and spies, and we will confront them with force," said Cmdr. Hossein Hamedani of the Revolutionary Guard, according to the official Islamic Republic News Agency.

Thousands of security personnel lined Revolution Avenue, allowing the march to continue but preventing the marchers from congregating in Azadi Square -- considered a rallying point by opposition groups.

"You can't take two steps without running into security personnel," one witness said. "They're all over the place."

Several protesters who were diverted by police to side streets were beaten with batons and gassed by security officers who were waiting at those locations, witnesses said.

At least 40 people were detained as riot police on motorcycles continue to patrol Tehran, one witness said.

Video uploaded to YouTube showed throngs of demonstrators marching, burning posters of Ayatollah Ali Khamenei and in one instance beating a man who appeared to try to remove a poster from the hands of protesters.

Other YouTube video showed police in riot gear pursuing dozens of people running away from the baton-wielding officers.

CNN cannot independently verify the authenticity of the videos and witnesses declined to be named for fear of retribution.

Reporting from Iran proved extremely difficult Monday, as foreign journalists were denied visas, accredited journalists living in the country were restricted from covering the demonstrations and internet speeds slowed to a crawl in an apparent attempt to both limit protest organizing and restrict information from being transmitted out of the country.

The Iranian government had blocked the homes of opposition leaders after they called for rallies in support of the uprising in Egypt that toppled a near three-decade-long rule of Egyptian President Hosni Mubarak.

About 200 protesters -- some of whom chanted "death to Khamenei" and "death to the dictator" -- set fire to several trash bins in the capital city and threw rocks at security forces, who tried unsuccessfully to subdue them, witnesses said. The chanting protesters apparently were referring to Ayatollah Ali Khamenei, Iran's supreme religious leader.

Clashes also erupted in front of Tehran University, where security forces dispersed crowds by firing tear gas and paint-ball guns, a witness said.

"There needs to be a commitment to open up the political system," U.S. Secretary of State Hillary Clinton said Monday after a meeting with U.S. House Speaker John Boehner.

Clinton said the crackdown is "an indictment of the hypocrisy of the Iranian regime" that constantly "hailed" the protests in Egypt but "once again illustrate their true nature."

The head of Iran's National Security Council and other Iranian authorities had earlier compared "the Egyptian Revolution with the victory of Iran's Islamic Revolution," according to Iranian state-run media.

Last week, the Iranian government rounded up activists after opposition leaders Mehdi Karrubi and Mir Hossein Moussavi called for supporters to gather at Azadi Square -- the site of mass protests by Iran's opposition movement after the disputed 2009 presidential elections.

Security forces also blocked roads leading to Moussavi's home, his opposition website, Kaleme, reported. The website also said phone lines and cell phone service to the area had been cut off.

Plainclothes security forces blocked Moussavi's wife, Zahra Rahnavard, from leaving their home Monday, according to Kaleme and another opposition website, Saham News.

Surveillance cameras installed outside Karrubi's home have been stolen and destroyed, Kaleme reported.

About 50 riot police on motorcycles were seen near Azadi Square, while 100 more were stationed at Ferdowsi Square in the city center.

Iranian authorities had warned against holding the rally, according to state-run media.

"They are scared," then-U.S. press secretary Robert Gibbs said Friday, hours after Mubarak stepped down from power.

"That's why they threatened to kill anybody that tries to do this," Gibbs said of the Iranian government. "That's why they have shut off all measure of communication."

Over the weekend, Iranian authorities blocked the word "Bahman" -- the 11th month on the Iranian calendar -- from internet searches within the country, according to an opposition website.

The measure appeared to be an effort by Iranian authorities to obstruct access to several opposition websites promoting the rally using the name of the month to draw mass demonstrations, Saham News reported Saturday.